Even when stressed below its ultimate strength, repeated stress on a mechanical part can cause failure due to fatigue. Fatigue life has been studied in many materials, and particularly those materials which are employed in the building of structures which are intended to have long life, but which are designed close to the ultimate strength limit. Such structures are subject to fatigue failure when repetitive stresses below the ultimate strength are encountered. Fatigue strength is principally a function of the materials, the manner in which the material has been treated, temperature, the amount of stress, and the number of cycles.
In the various fields of electronics, there is a wide variety of materials used in physical conjunction with each other. Semiconductor materials are connected by metal connectors and are mounted on non-conductive bases, such as ceramic or filled organic bases. Each of these structures has different thermal expansion properties and, as a result of thermal cycling, fatigue stresses are created and fatigue failure can take place. It is important in many electronic structures to be able to predict fatigue failure so that a part can be replaced before failure. When electronic assemblies are in operation, the remaining useful life is unknown, but many fail due to separation of different parts due to repeated thermal stress. A thermal fatigue testing method and apparatus therein will allow monitoring of leftover life.